Aerial photographs, topographic maps and the like frequently require the finding of information such as the distance and azimuth between selected points, scale, and the position of true north.
A commonly used measuring device suitable for this purpose is identified as an Aerial Photo Scale-Protractor. This device is a single, elongated, rectangular, transparent sheet having a protractor at one end with a North indicia at 0, cross hairs at 0 and 90 degree intervals corresponding to east, west and south and with scale numbers from 0 to 180 in both directions from 0. A plurality of different distance scales are arranged along the sheet. The procedure for using this device is to center the cross hairs on a first point, locate the N-S axis parallel to a reference line (true north), place a straight edge to intersect the two points, read the angle on the protractor scale, and measure the distance between the two points on the distance scale. Several disadvantages of this device are that:
The user must simultaneously keep the cross hairs centered over a first point, keep the north-south axis parallel to the reference line (true north), and arrange a straight edge so that it passes through both points to find the azimuth. The alignment with the reference line (true north) is typically done by a visual alignment regardless of the distance separating the north-south axis from the reference line. The distance between the points must be found in a separate step, during which all of the above alignment is negated.
This procedure must be repeated between every two points, i.e. between points one and two, two and three, etc. If the position of true north, or the scale of the photo is unknown, it is very difficult or impossible to use this device. In summary, this device is somewhat tedious to use, has great potential for inaccuracy, is impractical and is essentially useless without knowing the position of true north and/or the scale of the aerial photo.
A more general prior art drawing and drafting tool sold as the CRAYOLA.RTM. measure maker is comprised of two opaque, rigid arms that are pivotally connected together and with a protractor arranged for indicating the relative angular position between the two arms. This device is not specifically adapted to measure and plot the above discussed parameters on topographic maps and aerial photos.